1. Field of the Invention
This invention relates to a process and articles prepared thereby. More specifically, this invention is directed to the enhancement in electret behavior of poly(vinylidene fluoride) by phase transformation of said polymer from the alpha to gamma form, and the utilization of such materials in sound production and transmission devices.
2. Description of the Prior Art
The existence of at least two crystalographic phases of poly(vinylidene fluoride) has been apparent since the initial studies of this polymer. Crystallization from the melt was found to produce spherulites of the alpha (.alpha.) phase in which the chain backbone assumes a trans-gauche-trans-gauche' (TGTG') conformation. The orientation of these melt crystallized samples transform the TGTG' alpha phase crystals to a planar zig-zag conformation called the beta (.beta.) phase. This beta phase can also be formed by casting from selected solvents and by high pressure crystallization methods.
In addition to the alpha and beta phases, several investigators have suggested the existence of a third polymorph of poly(vinylidene fluoride); although, the evidence that has been offered to confirm the existence of this proposed gamma (.gamma.) phase is both sparce and inconclusive. Cortili and Zerbi were the first to suggest the possibility of this third phase on the basis of small changes in the ir spectra of differently prepared samples of the zig-zag phase. Doll and Lando subsequently found that high pressure crystallization of poly(vinylidene fluoride) produced a material with a melting point 25.degree. C above what has been previously observed for either the alpha or beta phase, J. Macromol. Sci. Phys. 2(2), 219 (1968).
Subsequent work by Gal'Perin et al described the preparation of a new polymorph of poly(vinylidene floride). They found that films cast from dimethylformamide or dimethylsulfoxide yielded the new gamma phase and that a more developed form of this polymorph could be obtained by annealing these films at 458.degree. K. They also noted that this phase could be made by melt crystallizing low molecular weight (Mn&lt;10.sup.5) poly(vinylidene fluoride) between 443.degree. and 458.degree. K. Attempts to orient this gamma form triggers a transformation to the beta phase similar to the strain induced alpha to beta transition. This observation is consistent with the results of Doll and Lando on oriented pressure crystallized poly(vinylidene fluoride) and suggests that they were successful in their original attempt to produce the gamma phase of poly(vinylidene fluoride).
It has also been previously shown that the slow growth rate associated with high temperature crystallization permit concurrent formation of a different phase of poly(vinylidene-fluoride) J. App. Phys. 46 (10), 4136 (1975). These slow crystallizations are shown to produce unique morphological structures with unusual melting behaviors. It has also been shown that a time-and-temperature-dependent crystal transition from the alpha to the gamma phase form can be induced in a high temperature crystallization region to produce spherulites which melt 15.degree.-20.degree. K above the melting point of the original alpha phase. This latter transformation exhibits nucleation and propagation characteristics which in some regions can compete with the normal growth of the alpha phase to produce unique "wagon wheel" spherulitic structures.
The prior art also discloses the addition of certain additives to poly(vinylidene fluoride) to control and/or eliminate the formation of large spherulites within this polymer, U.S. Pat. Nos. 3,701,749 and 3,850,900. Since both of the above patents deal with crystallization of this polymer by conventional techniques, only the alpha form of the polymer is produced.
In all the prior art processes described in the articles referred to hereinabove, the preparation of the gamma phase of poly(vinylidene fluoride) requires resort to elaborate process conditions which involve extended reaction time. Even when these conditions are satisfied, the yield is often extremely low. Accordingly, it is the object of this invention to remedy the above, as well as related, deficiencies in the prior art methods for preparation of gamma poly(vinylidene fluoride).
More specifically, it is the principle object of this invention to provide a process for rapid and efficient conversion of the alpha form of poly(vinylidene fluoride) to its corresponding gamma phase.
It is another object of this invention to provide a process for the conversion of the alpha and/or beta form of poly(vinylidene fluoride) to its corresponding gamma phase at higher yields than available in prior art systems.
It is yet another object of this invention to provide a process for the conversion of the alpha form of poly(vinylidene fluoride) to its corresponding gamma phase without resort to elaborate process conditions or extended reaction times.
Still yet another object of this invention is to provide a process for reduction in the alpha phase in poly(vinylidene fluoride) samples.
Additional objects of this invention include the utilization of the gamma form of poly(vinylidene fluoride) as electrets in sound production and sound transmission devices.